The present invention relates generally to wiregrid polarizers, and more specifically to a new array pattern of individual wiregrids for use in imaging polarimeters.
Remote sensing applications can make use of the optical polarization characteristics of a scene to enhance target detection and discrimination. Imaging polarimeters utilize polarizing arrays positioned in front of a focal plane array of detector pixels to extract polarization information from the optical scene.
An integrated microgrid imaging polarimeter includes a repeating pattern of wiregrid polarizers bonded to a focal plane array. A wiregrid polarizer can be made as a layer of very thin ruled aluminum wires sandwiched between two glass windows. The most common microgrid arrangement is a 2×2 repeating pattern of so-called analyzer cells. A typical 2×2 array is shown in FIG. 1. The lines in each cell correspond to the wiregrid element orientation. This specific pattern has been the standard since 1994.
Raw microgrid data are used to infer Stokes parameter images. The power spectrum of a raw microgrid image consists of a high bandwidth unmodulated S0 image spectrum surrounded by the low bandwidth spectra of S1 and S2. The S1 and S2 spectra are modulated out to the Nyquist frequency in the direction of the principal axes of the array. The connection between this Fourier analysis of modulated polarimeters as linear systems and the more widely known data reduction matrix was shown in 2012. The microgrid spectrum is similar to Color Filter Arrays (CFAs) used to extract color information for digital cameras, where a mosaic of tiny color filters are placed over the pixel sensors of an image sensor to capture color information, in that the high bandwidth spectrum centered at a DC-centered array corresponds to the luminance image and the low bandwidth modulated portions of the spectra correspond to chrominance.
Despite the long-term use of the standard 2×2 array and its pattern of different polarization directions, there is still a need for greater resolution and image quality.